dispatch: Code generate api_exec.c.

This patch adjusts makefiles to cause src/mesa/main/api_exec.c to be
generated using src/mapi/glapi/gen/gl_genexec.py.  There should be no
functional change.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index 53e5ee8..11e034f 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -441,6 +441,14 @@
     command = python_cmd + ' $SCRIPT -f $SOURCE > $TARGET'
     )
 
+# The api_exec.c file is generated from the GL/ES API.xml file
+env.CodeGenerate(
+    target = 'main/api_exec.c',
+    script = GLAPI + 'gen/gl_genexec.py',
+    source = GLAPI + 'gen/gl_and_es_API.xml',
+    command = python_cmd + ' $SCRIPT -f $SOURCE > $TARGET'
+    )
+
 # We also depend on the auto-generated GL API headers
 env.Depends(mesa_sources, glapi_headers)